1. Field of the Invention
The present invention pertains to the art of sensing devices and, more particularly, to a self-locating mounting apparatus in the form of headgear for accurately positioning sensors on a subject, such as for medical purposes.
2. Discussion of the Prior Art
There are a number of applications in which it is necessary to place sensors on the scalp of a human subject at defined locations relative to the head. One principle application is in the recording of electrical brain activity via an array of discrete bioelectrodes or electroencephalography (EEG) sensors that measure the scalp surface potential at specific locations. The standard set of measurement locations is known as the International 10/20 electrode placement scheme and is defined based on linear ratios tied to specific physiological markers, such as the nasion (intersection of the frontal and two nasal bones, of the human skull), the inion (the most prominent projection of the occipital bone at the lower rear part of the skull), and the pre-auricular points (adjacent the ears). More recently, subsets of sensor positions have been defined to be used in measurement of specific cognitive states, e.g., cognitive overload. In this case, sensors can be infrared sensors to detect brain blood oxygen content or biopotential electrodes. Other sensors, such as accelerometers and/or impact sensors, can be placed at specific locations on the head to quantify forces on the head resulting from collisions and explosions that could lead to brain trauma.
Existing methods to position sensors and other objects on the head of a subject involve careful measurements by trained personnel. Measurement reference points such as the nasion are used first to define starting points for measurement of head circumference and the distance from the front to the back of the head across the centerline. Desired positions are then defined via fraction distances along these key measurement lines or along lines that connect points on the key measurement lines. The technical challenge is that the size and shape of the human head can vary significantly. For example, the circumference along the nasion and inion points typically varies by 8 cm in adults and the distance across the top of the head typically varies by 4 cm front to back. The simplest solution that has been tried is to hold sensors in an entirely elasticized structure that can expand in all directions, such as a latex cap. See, for example, the prior art arrangement depicted in FIG. 1 herein, showing a stretchable (elastic) cloth hat body 2 including electrodes 4. One such cap is taught in U.S. Pat. No. 4,967,038 entitled “Dry Electrode Brain Wave Recording System” filed Aug. 25, 1989, which is incorporated herein by reference.
When working with sensor arrangements adapted to fit multiple head sizes, the change in head dimension is ideally divided equally across all the sensor positions, resulting in an equal relative expansion of the distance between sensors. However, in practice, expandable mounting apparatuses such as hat body 2 shown in FIG. 1 become caught on parts of the head and the expansion is uneven. In addition, sensors in mounting apparatuses which are entirely elasticized are prone to become tilted, i.e., they do not sit flat against the head, due to the dragging force generated when the structure is put on. The problem of tilting is increased when the objects require application of a force normal to the head. In this case, it is better to attach the sensors or objects to the mounting apparatus via their outer surface (with respect to the scalp) so as to produce a component of the force in the direction normal to the head. However, attaching the objects in a plane at a considerable distance off the scalp increases the moment that acts to rotate the objects in direction proportion to distance.
Accordingly, there exists a need for a mounting apparatus to hold objects such as sensors against the scalp wherein the apparatus expands when placed on a subject's head in order to fit varying head sizes, but which effectively locates the sensors at a desired position relative to the head and prevents rotation of objects when the mounting apparatus is in use.